In each Indiviual profile there is a tab called 'Identifiers'.
This page keeps track of id:s a customer profile have in different external systems so it knows it is the same profile.
For example if a guest first make a reservation filling in his email and next time logs in using facebook this page will be the link for the system to know that it is the same customer.
If you have a channel manager integration it will also import any customer ids that the different booking channels provides.
